Known vertical air to air heat pipe heat exchangers rely on gravity to return the condensed working fluid from the condenser to the evaporator. For this reason, the condenser must always be located above the evaporator. This works well for seasons when the air stream in the lower duct is hotter than the air stream in the higher duct. However, the known vertical air to air heat pipe heat exchangers do not transfer any energy from the condenser to the evaporator if the evaporator is located above the condenser, as the condensed working fluid from the condenser cannot be returned to the evaporator. Consequently, known vertical air to air heat pipe heat exchangers only work “one way,” meaning they can only transfer heat between the streams when positioned in a gravity aided position.
In order to overcome this problem, more complex and expensive heat exchangers have been developed having two separate split coils with a pump provided there between to pump the working fluid between the split coils.
It would, therefore, be beneficial to provide a two phase air to air heat exchanger in which a pump is provided to allow the direction of the heat flow through the heat pipes to be variable depending upon the ambient air conditions.